Abstract

Healthy subjects were tested in two experiments to examine the effects of lateralized cues on line bisection and landmark judgments. The studies were designed to investigate whether bisection and landmark biases induced by cueing are simply a result of a direct perceptual lengthening of the cued part of the line caused by the fact that the cue is visible, thus creating a composite 'line plus cue' or whether cueing induces an attentional bias operating on judgments of spatial extent by either reducing the magnitude of the parts of the stimulus receiving 'less' attention or magnifying those parts receiving 'more'. Lateralized cues were either visible letter cues or invisible marks drawn with a leadless pencil either by the subject him/herself (Experiment 1) or the experimenter (Experiment 2). Comparable to a previous study (Mattingley, Pierson, Bradshaw, Phillips and Bradshaw, 1993, Neuropsychologia, 31, 1201-1215), the first experiment showed that only visible cues affected line bisection and landmark judgments thus favouring the perceptual explanation. The second study, however, revealed bisection and judgment biases for invisible as well as visible cues with the attended part of the line appearing subjectively longer. These results indicate that attentional modulations can increase the salience of a line in a similar vein to physical changes. It seems likely that the requirement of a motor response in Experiment 1 cancelled out all attentional properties supposedly induced by placing that invisible cue.

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